Tray hydraulics

The froth on the tray is a turbulent mass of usually liquid-continuous fluid with vapor dispersed in the form of small bubbles. The various designs of tray vapor openings attempt to maximize vapor dispersion by generating the smallest possible bubbles. The froth is where mass transfer takes place between the vapor and the liquid. Mass transfer and tray efficiency are enhanced by creating the largest possible interfacial area. 

The starting point for the bottom energy model is the general tray heat balance Eqn. (16.26), adjusted for the bottom. The heat is not supplied by the vapor flow from the tray underneath but by a heating source, which can be mounted internally or externally. 

When the heat requirement by the tray capacity and the heat losses are negligible with respect to the heat supply of the vapor flow, then the equation for the vapor flow Fo for a tray (Eqn. (16.34)) can be simplified further as indicated in the sequel. When the heat supply has a heat capacity, this can often be described by a first-order lag behavior with a time constant 

When the heat is supplied by steam, then the supply can be described by  

The hold-up of a tray Mi consists of the liquid mass on the tray Md, downcomer A/, and the vapor hold-up A/f (Fig. 16.5). 

As stated before the vapor hold-up has been neglected. 

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Where is nonzero only for tray N, y and x refer to the light component only such that the corresponding mole fractions for the heavy component are (1 — y) and (1 — x), L and are the initial steady-state values, and P is a constant that depends on tray hydraulics. 

The Souders-Brown correlation considers entrainment as the controlling factor. For high liquid loading situations and final design, complete tray hydraulic calculations are required. 

For final design, complete tray hydraulic calculations are required. 

Fair, J. R., Tray Hydraulics-Perforated Trays, Chap. 15 in... 

Bolles, W. L. (1963) Tray hydraulics bubble-cap trays, in Design of Equilibrium Stage Processes, Smith, B. D. (McGraw-Hill). 

More rigorous relationships can be obtained from the detailed tray hydraulic equations to include the effects of vapor rate, densities, compositions, etc. We will assume a simple functional relationship between liquid holdup and liquid rate. 

ASSUMPTIONS CONSTANT RELATIVE VOLATILITY, EQUIMOLAL OVERFLOW, THEORETICAL TRAYS, SIMPLE LIQUID TRAY HYDRAULICS... 

Fischer CH, Quarini GL. Three-dimensional heterogeneous modeling of distillation tray hydraulics. Proceedings of AIChE Annual Meeting, Miami Beach, 1998. 

Now that I ve confused you with the preceding description (though I believe it to be very much to the point in a simple way), I present a very easy way to understand tray hydraulics. Only three inputs are necessary to design any fractionation tray ... 

Because only three values—diameter, downcomer width, and number of passes—define essential tray hydraulics, you can run many trials in a short time. Simply input values into the computer program or execute hand calculations using different widths or passes, seeing if you can get a good design with the chosen diameter. You can even more quickly rate an existing tray, since all trays are designed on the basis of these three rules. 

Refer to the earlier section of this chapter concerning tray hydraulics. Please note that the balance of the tray hydraulics fixes all downcomer areas when one side downcomer area is determined. This is true for any multitray liquid pass 2-pass, 3-pass, or 4-pass and greater. Please refer again to Fig. 3.1, which diagrams these multitray liquid passes. The following equations are therefore presented ... 

Correct the HHDS sieve tray hydraulic gradient with the beta factor (froth correction) ... 

Key 1 - without holdup 2-with holdup 3 - with tray hydraulics... 

Tung, L.S. and Edgar, T. F., "Development and Reduction of a Multivariable Distillation Column Model with Tray Hydraulics,"... 

Determine the effects of the physical properties of the system on column efficiency. Tray efficiency is a function of (1) physical properties of the system, such as viscosity, surface tension, relative volatility, and diffusivity (2) tray hydraulics, such as liquid height, hole size, fraction of tray area open, length of liquid flow path, and weir configuration and (3) degree of separation of the liquid and vapor streams leaving the tray. Overall column efficiency is based on the same factors, but will ordinarily be less than individual-tray efficiency. 

Determine the effects of tray hydraulics on the efficiency. Tray hydraulics will affect efficiency adversely only if submergence, hole size, open tray area, and weir configuration are outside the recommended limits outlined in the previous example. Since that is not the case, no adverse effects need be expected. 

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